1. Field of The Invention
The present invention is broadly concerned with leak integrity testing apparatus for use in testing liquid-conveying conduits, and especially multi-sectional pipe strings, used in various industrial and oil well applications. More particularly, the invention pertains to such apparatus, and especially a frangible forward acting rupture disc forming a part thereof, wherein the rupture disc is of concavo-convex design and has a non-circular, continuous score line formed in the concave face thereof. The score line is of continuously varying depth around the circumference thereof and includes an outwardly projecting cam segment which is of greatest depth that ruptures first and is in direct opposition to a lesser depth score line portion forming a hinge area for the burst region of the disc preventing separation thereof from the flange portion of the disc upon disc rupture. The apparatus has particular utility for use in testing the integrity of the connection between the sections of oil well tubing strings, but also may be used in petroleum refining and petrochemical operations, as well as other uses in which a liquid is conveyed under pressure through a pipe or conduit.
2. Description of the Prior Art
In order to place an oil well in service, an elongated, sectionalized tubing string is lowered into the well casing, with the tubing string housing a sucker rod and pump assembly. In deep wells, the tubing string may extend thousands of feet from grade down to an oil formation. Leaks in the joints between string sections have a significant impact on pumping efficiency and oil well production. In the past, it has sometimes been necessary to remove the pipe string, locate joint leaks, and repair the string. This can represent a very substantial expense both in terms of repair costs and well downtime.
U.S. Pat. No. 5,996,696 describes a method and apparatus for leak integrity testing of oil well tubing within the well casing, thus eliminating the need for string removal for such testing. The apparatus of the ""696 patent includes a housing interposed between string sections (usually adjacent the lower end thereof close to the pump and well formation). The housing is equipped with a metallic rupture disc in closing, flow-blocking relationship. When it is desired to test the string, predetermined fluid pressure is applied against the rupture disc. If the tubing string is sufficiently leak-free, the rupture disc will burst at or about the predetermined burst pressure. On the other hand, if substantial leaks are present, insufficient pressure will be developed within the string to burst the disc.
The preferred rupture disc design disclosed in the ""696 patent is a metallic, concavo-convex disc having a discontinuous score line formed in a face of the bulged rupture portion thereof. The discontinuity in the score line serves as a hinge region for the disc. While the apparatus and methods described in this patent represent a significant breakthrough in the art, it has been found that sometimes the disc design is not optimal from a performance standpoint. As can be appreciated, a useful rupture disc in this context must reliably burst at desired burst pressures, or inaccurate test results may be obtained. In addition, the disc after being ruptured must not fragment while still allowing full bi-directional liquid fluid flow therethrough.
The present invention overcomes the problems outlined above and provides an improved leak testing apparatus for liquid-conveying line applications, and especially oil well tubing strings, giving increased testing reliability. Broadly speaking, the apparatus of the invention includes a tubular holder adapted for coupling to an oil well tubing string with a rupture disc within the holder in normally closing relationship to the central passage through the holder. The disc includes an outer annular peripheral flange portion having a first annular face, and an opposed second annular face and an inner circular margin, and an inner concavo-convex, generally hemispherical bulged region inboard of and projecting away from the first annular face of the flange portion. The bulged region has opposed convex and concave faces and is provided with an outer circular margin and a unitary, annular, transversely arcuate transition region integral with and joining the outer circular margin of the bulged region with the inner circular margin of the flange portion.
The bulged region of the disc is provided with a continuous non-circular score line in the concave face thereof adjacent the outer circular margin of the bulged region in proximal spaced relationship to the transition region. The score line has a semi-circular segment of major length, and a second generally curvilinear score line cam segment of minor length with respect to the length of said semi-circular score line segment. The cam score line segment projects outwardly of the diameter of the major score line segment and is located closely adjacent and inboard of the transition region.
The score line varies in depth along the length thereof and is of greatest depth along the cam score line segment and of shallowest depth in substantially direct opposition to the cam score line segment thereby defining a hinge portion of the score line.
The disc is openable under the influence of liquid overpressure directed against the concave face of the disc to allow flow of the liquid past the disc. The cam score line segment is of a depth relative to the remaining portion of the score line to cause the cam score line segment to rupture first under liquid pressure while the major score line segment is of a varying depth which allows the burst region to swing to a fully opened rupture position without severing of the hinge portion of the score line. Upon rupture of the bulged region and full opening of the disc, the edge of the central burst region is disposed in generally perpendicular relationship to the flange portion of the disc.
The tubular holder is provided with sidewall structure configured to generally complementally receive and allow sufficient full opening of the rupture disc under liquid overpressure to thereby permit full flow of liquid through the tubular holder upon rupture of the disc. It has been found that discs of this character provide improved testing results, as compared with prior designs, while allowing full bi-directional flow of liquid through the holder after rupture of the disc.
The provision of the continuous, non-circular score line on the concave face of the disc permits reliable disc rupture and opening owing to the fact that when rupture occurs, the adjacent interconnected score line-defining wall surfaces of the disc separate from each other. This is to be contrasted with the more usual situation where the score line(s) on rupture discs are formed in the face thereof remote from contact with the product and/or pressure. In such a situation, the score line-defining wall surfaces of the disc move toward each other during rupture.
The curvilinear cam portion of the score line, and which is of greatest depth, projects beyond the circumference of the main circular portion of the score line and acts somewhat as a cam or lever to assure reliable opening of the disc at pressures of the order of about 1500 psi to about 2000 psi. Opening pressures of that magnitude are difficult to achieve in discs having a nominal burst diameter of the order of 2 to 3 inches, which are required to block the internal diameter of conventional oil well pipe tubing strings, without the extended, deepest depression portion of the score line which projects beyond the circumference of the major circular portion of the score line.